This invention relates generally to wrap spring clutch and brake devices and more particularly, it relates to a wrap spring clutch/brake apparatus which includes a helper clutch mechanism for supplying increased frictional torque at lower rotational speeds, thereby eliminating any error in the starting/stopping point of a load.
As is known to those skilled in the art, wrap spring clutch/brake devices are used for starting and stopping a load so that it stops at the same desired point without cumulative error. In such clutch/brake devices, two control tangs are provided to hold either the clutch spring or the brake spring open or unengaged with a stop collar. When the clutch and brake control tangs rotate with a continuously rotating hub input, the hub input and a shaft output are positively engaged by the clutch spring and the brake spring is disengaged. When the brake control tang is locked by the stop collar, the brake spring wraps down to engage the shaft output to a stationary brake hub. At the same time, the clutch spring unwraps slightly allowing the hub input to rotate freely.
Since there is a spring differential between the time of disengagement of the clutch spring and the engagement of the brake spring, the shaft output connected generally to a load will continue to rotate due to the spring differential. Therefore, the amount of rotation of the shaft output will be dependent upon the load. Accordingly, these prior art clutch/brake devices have the unfortunate aspect of being very inertia sensitive and that inertia must be "tuned" for the device to function accurately. In other words, the clutch/brake device will operate accurately for only a particular rotational speed for a given load. When such clutch/brake devices are driven at variable speeds for the given load, variable inertia will be present and thus, the accurate positioning operation is significantly reduced.
At very low speeds, the given load will have a very low inertia which may prevent the output shaft from continuing to rotate after the disengagement of the clutch spring and before engagement of the brake spring (spring differential). As a result, the load will not be starting and stopping at the same desired position. In order to overcome this problem, the present invention provides a helper clutch mechanism which supplies increased frictional torque at lower rotational speeds to the shaft output, thereby insuring accurate starting and stopping of a load at the same desired position without cumulative error.